Thymocyte transformation is caused by combinations of particular genetic abnormalities, leading to a dependence on specific oncogenic pathways that are uniquely required for cell proliferation, survival and clonal expansion of leukemic T-cells. Identifying such pathway dependence and critical genetic abnormalities in T-cell acute lymphoblastic leukemia (T-ALL) will reveal promising candidates for the development of novel targeted therapies for this disease. Our preliminary data was generated with the highly innovative technology of a loss-of-function RNA interference screen, and we found that loss of TYK2, a JAK family tyrosine kinase, was lethal to leukemic cells from T-ALL cell lines and primary T-ALL samples. We discovered that the TYK2 protein is constitutively activated in manyof T-ALL cell lines, and that these cells are sensitive to a small molecule JAK/TYK2 inhibitor through the induction of apoptosis. These results imply a specific pathway dependence on this gene and an essential role for the TYK2 signal transduction pathway by cells from the majority of T-ALLs. The objectives of this proposal can be summarized as: (i) to delineate the components of the TYK2 pathway in T-ALL and the basis for the dependence of T-ALL cells on this pathway for cell survival, and (ii) to identify the most promising small molecule TYK2 inhibitors in preclinical models as a first step toward the long-range goal of developing a novel therapeutic approach in T-ALL. These objectives will be achieved through three specific aims. In Aim 1, we will clarify the molecular mechanisms that activate the TYK2 pathway in T-ALL. In Aim 2, we will evaluate the effectiveness of lead compounds that inhibit TYK2 in vitro. In Aim 3, we will study the in vivo sensitivity of T-ALLcells to prioritized small molecule TYK2 inhibitors in murine orthograft and primagraft models. Dr. Takaomi Sanda is an instructor in the Department of Pediatric Oncology at the Dana-Farber Cancer Institute (DFCI) working under the mentorship of Dr. Thomas Look, an internationally recognized leader in the field of leukemia research. Building on Dr. Sanda's medical and scientific training in Japan, he has uncovered and is now defining the role of tyrosine kinases in the proliferation and survival of cancer/leukemia cells. Dr. Look's proven mentorship coupled with the rigorous and nurturing scientific environment offered by the research community at DFCI and affiliated institutions offer the maximal opportunity for Dr. Sanda's success during the award period as an instructor and in his transition to becoming an independent investigator.